The present invention relates to a rotary shear line in which sheet stock is fed by a feeder to a rotary shear for fly cutting by its rotating cutters.
FIG. 1 shows a conventional rotary shear line, in which sheet metal or similar sheet stock 11 is leveled by a leveler 12 and is then fed to a rotary shear 13. A line speed V.sub.L (i.e. the feed rate of the sheet stock 11) is set in a driver 14, by which a motor 15 is driven to rotate the leveler 12 through a distribution gear 16. The rotary shear 13 is driven by a shear motor 17. A length measuring roll 18 is held in rotary contact with the sheet stock 11 and drives an encoder 19 to yield therefrom pulses corresponding to the travel of the sheet stock 11. The pulses thus obtained are applied to a numerical controller 21. An encoder 22 is driven by the shear motor 17 to generates pulses corresponding to the rotational movement of cutting edges 23 of the rotary shear 13, and these pulses are also provided to the numerical controller 21. The numerical controller 21 controls the shear motor 17 in accordance with a preset cutting length L and the pulses input thereto.
FIG. 2 shows another conventional rotary shear line, in which the sheet stock 11 delivered from the leveler 12 is fed by a feeder 24 at a constant feed rate. The line speed V.sub.L (i.e. the feed rate of the sheet stock 11) is set in a driver 25, which drives a feeder motor 26 for driving the feeder 24. The same line speed V.sub.L is also set in the driver 14, which controls the motor 15 in accordance with the amount of loop 27 of the sheet stock 11 between the leveler 12 and the feeder 24; in this example, the motor 15 is controlled so that the amount of loop .OMEGA. remains substantially constant at all times.
In the rotary shear line depicted in FIG. 1 the leveler 12 may sometimes be preceded by a plating, annealing or similar processing stage, and in the rotary shear line shown in FIG. 2 the feeder 24 may sometimes be preceded by a processing stage different from the leveler 12. At any rate, the rotary shear line operates on the following principle.
The numerical controller 21 controls the rotary shear 13 through the shear motor 17 so that the upper and lower cutting edges 23 are rotated 360 degrees for each preset feed of the sheet stock 11 and mesh with each other at a distance of the preset cutting length from the forward end of the sheet stock 11 and so that the peripheral speed of each cutting edge 23 is synchronized with the feed rate of the sheet stock 11, i.e. the line speed V.sub.L.
The rotating speed V.sub.M of the shear motor 17 varies with preset cutting lengths as shown in FIG. 3A, B and 3C. FIG. 3A shows the case where the preset cutting length is short; in this instance, the shear motor 17 is accelerated above the line speed V.sub.L and then decelerated, after which while its rotating speed remains equal to the line speed V.sub.L, the shearing work takes place in such a period as indicated by the pair of arrows. FIG. 3B shows the case where the preset cutting length is medium; in this instance, the shear motor 17 is decelerated a little below the line speed V.sub.L and then accelerated, after which the shearing work takes place while its rotating speed remains equal to the line speed V.sub.L. FIG. 3C shows the case where the preset cutting length is long; in this instance, the shear motor 17 is decelerated below the line speed V.sub.L and then accelerated, after which shearing work takes place while its rotating speed remains equal to the line speed V.sub.L.
In FIG. 4 the curve 28 shows a line speed V.sub.L vs. cutting length L characteristic which represents the performance of the conventional rotary shear line. The inside of the curve 28 (the hatched side) is the range in which sheet stock can be sheared. The locus of each cutting edge 23, which is formed when it is rotated 360 degrees, does not necessarily become a perfect circle according to the rotary shear used; in which case, however, the circumferential length corresponding to that of the cutting edge formed by its 360.degree. rotation can be obtained by conversion. In the illustrated example, the above-mentioned circumferential length is .XI.D=L.sub.SO =700 mm; the rated peripheral speed of the rotary shear 13 is 200 m/min; the rated feed rate of the feeder 24 is 240 m/min; the rated acceleration of the rotary shear 13 is ##EQU1## the rated acceleration of the feeder 24 is ##EQU2## and the settling time of the rotary shear 13 and the feeder 24 (in which the rotational speed of the cutting edges 23 of the rotary shear 13 and the feed rate of the sheet stock 11 by the feeder 24 are gradually synchronized with each other and the shearing work is effected immediately before the end of the settling time) is 0.1 sec or more.
Where the cutting length L is shorter than L.sub.SO =700 mm, the shear motor 17 must be accelerated above the line speed V.sub.L by a value corresponding to the length l=L-L.sub.SO short of the circumferential length of the locus of each cutting edge 23 (the length, l, will hereinafter referred to as an adjustment length) as shown in FIG. 3A. The torque necessary for accelerating the shear motor 17 is the product of inertia and angular acceleration. The motor torque is limited, whereas the mechanism of the rotary shear 13 for cutting sheet stock through utilization of large inertia of the shear itself is inevitably heavy, because it must yield large shearing force and withstand shocks of the shearing work. This imposes appreciable limitations on the acceleration of the rotary shear 13. On this account, as the cutting length L decreases, the line speed V.sub.L must be reduced sharply as indicated by the curve 28 in FIG. 4. For instance, when the cutting length L is 300 mm, an appreciable amount of time is needed to accelerate and decelerate the shear motor 17 by an adjustment length of 400 mm as shown in FIG. 5A. In this case, since the sheet stock 11 is fed 300 mm in the period of time which is the sum of the acceleration and deceleration time and the settling time T.sub.S, the line speed V.sub.L therefore is very low as depicted in FIG. 5A. The area of the obliquely hatched region represents the adjustment length and the area of the horizontally hatched region the cutting length L.
Where the cutting length L is, for example, 600 mm, however, the shear motor 17 needs only to be accelerated and decelerated for a time corresponding to an adjustment length of 100 mm as depicted in FIG. 5B. In this instance, since the sheet stock 11 must be fed as long as 600 mm throughout the acceleration and deceleration time and the settling time, the line speed V.sub.L is far higher than in the case of FIG. 5A.
Where the cutting length L is equal to .XI.D=L.sub.SO =700 mm, the shear motor 17 need not be accelerated and decelerated, and hence the line speed V.sub.L can naturally be increased as desired, but it is limited by the rating of the shear motor 17 or the feeder motor 26. In the case of the curve 28 in FIG. 4, the line speed V.sub.L is limited by the rating of the shear motor 17 and is 200 mm/min.
Where the cutting length L is a little longer than .XI.D=L.sub.SO =700 mm, that is, where the adjustment length l=L-L.sub.SO is small, the shear motor 17 is decelerated below the line speed V.sub.L by the adjustment length l as depicted in FIG. 5C, but since this deceleration is slight, the line speed V.sub.L may be held high. Where the adjustment length l is large to some extent as shown in FIG. 5D, however, if the line speed V.sub.L remains high, the time for acceleration and deceleration of the shear motor 17 must be extended to adjust the length l, and consequently, no sufficiently long settling time can be provided prior to cutting. On such an occasion, cutting inevitably takes place before substantially complete synchronization is established between the rotating speed of the cutting edges 23 of the rotary shear 13 and the feed rate of the sheet stock 11 by the feeder 24. This results in lowering of the cutting accuracy and leads to variations in the sizes of individual pieces cut off the sheet stock 11. To avoid this, it is necessary in this instance to increase the cutting length L and decrease the line speed V.sub.L as indicated by the curve 28 in FIG. 4. By reducing the line speed V.sub.L to such an extent as to permit the shear motor 17 to stop as shown in FIG. 5E, the sheet stock 11 can be cut into desired length, no matter how long they may be, if only the stopping time of the shear motor 17 is selected long as depicted in FIG. 3C. Thus, the line speed V.sub.L becomes constant in the longer cutting length region as seen from the curve 28 in FIG. 4.
For the reasons given above, the V.sub.L -L curve of the numerically-controlled rotary shear line of any mechanism is, in principle, similar to the curve 28.
As described above, in the conventional rotary shear line the speed must be slowed down materially for cutting sheet stock into short lengths.
Where the line speed is so low that the flywheel effect of the rotary shear including the shear motor cannot be expected yet sheet stock cannot be cut only with the torque of the shear motor--in practice, this often occurs under restrictions on the manufacturing cost of the machine--, the rotary shear may sometimes come to a halt without cutting the sheet stock because of the low line speed, or even if the sheet stock can be cut, the rotary shear almost stops its rotation, with the result that the sheet stock being fed is blocked by the cutting edges and hence it curves into a hump.
For the same reasons as mentioned above, the sheet stock cannot be cut into desired medium and long lengths, either, when the line speed is low.
Also when the cutting length L is longer than the afore-mentioned circumferential length L.sub.SO of the locus of each cutting edge 23, the line speed V.sub.L must be decreased though not so much as in the case of the short cutting length.